Various metal treatment processes involving the exposure of metal workpieces, such as, tools and dies, cutting tools, castings, machined parts forgings and the like, to thermally controlled atmospheres having specific compositions that modify the chemistry of the workpiece and improve its physical properties, are well known. Illustrative examples of such processes include nitriding, carbonitriding, nitrocarburizing, and oxidation processes.
Nitriding processes typically involve exposing ferrous metal workpieces to heated ammonia derived atmospheres containing active nitrogen within a suitable furnace. The active nitrogen, usually derived from raw ammonia thermally decomposed within the furnace, diffuses into the workpieces' surface forming a nitrogen rich surface layer containing complex nitrides. See: U.S. Pat. No. 4,236,942.
Carbonitriding and nitrocarburizing typically involve ammonia derived atmospheres similar to nitriding except that the furnace atmosphere also contains active carbon that diffuses into the workpiece in addition to the nitrogen. See: U.S. Pat. No. 3,663,315.
Nitrided, carbonitrided and nitrocarburized workpieces display improved properties including greater hardness and enhanced wear, corrosion, and fatigue resistance making these processes useful in the production of metal cutting tools, machine parts, and the like.
The effectiveness of the foregoing processes is often diminished by prior processing steps that affect the workpiece surface, e.g. machining or polishing. Such surface processing may deactivate the surface to diffusion type metal treatments by producing surface oxides, surface carbides, deformations, or stresses that interfere with the diffusion of carbon and nitrogen. This shortcoming in the prior art can be overcome by the present invention preconditioning the workpiece in steam, air, humidified nitrogen, or humidified air atmospheres, as disclosed herein, to reactivate its surface so that treatment, e.g. nitriding, proceeds normally.
Another process according to the present invention for improving the corrosion resistance and wear characteristics, as well as, cosmetic appearance and ability to hold lubricant in metal workpieces is selective oxidation in wet or humidified atmospheres as hereinafter disclosed. Furthermore, this process is particularly useful for decreasing porosity and improving the compressive strength of powder and cast metals.
It is often desirable and sometimes required, to practice a combination of metal treatment processes on a single workpiece, e.g. preconditioning,--nitriding,-- selective oxidation, to facilitate effective treatment and impart the improved physical properties that result from each process. Clearly, it would be advantageous to practice each process of such combinations in a single apparatus. Specifically, this would reduce handling of workpieces to be treated, reduce the process time and reduce energy and equipment requirements.
Heretofore, it has not been practical to carryout many different metal treatment processes in a single furnace because at least one of the chemicals used in one process is incompatible with those used in a second process, and so on. The incompatible chemicals may combine or otherwise react to form extremely corrosive, poisonous, or noxious products that damage the furnace and other equipment and or present health hazards. For example, it has heretofore been impractical to practice processes employing ammonia derived atmospheres and processes employing moisture bearing atmospheres in the same furnace because water vapor and ammonia form corrosive combinations that rapidly destroys the metal surfaces it contacts, e.g., the furnace retort. It has not proven economically feasible to purge or clean conventional furnaces between various incompatible processes since this entails a great deal of labor and down time for the apparatus.